Carbon dioxide and the stomatal control of water balance and photosynthesis in higher plants. Progress report, July 1, 1990--June 30, 1992

The effect of glyphosate [N-(phosphonomethyl) glycine] on barley (Hordeum vulgareL.) and corn (Zea maysL.) shoot δ-aminolevulinic acid (ALA) production was examined by monitoring ALA content in the tissue and measuring incorporation of14C precursors into ALA and chlorophylla. Barley shoot ALA content was significantly decreased by 1 mM glyphosate after 9, 11, and 15 h of illumination. ALA production by treated barley shoots was 30 nmoles•g fresh weight-1•h-1at each interval tested, compared with 75 to 120 nmoles•g fresh weight-1•h-1for the control. In corn shoots, ALA content was reduced 32, 45, and 58% by 0.1, 1.0, and 10.0 mM glyphosate, respectively, after 12 h illumination. Incorporation studies with14C-glutamate,14C-α-ketoglutarate, and14C-glycine into ALA showed a 77, 92, and 91% inhibition, respectively, in barley shoots treated with 1 mM glyphosate. Incorporation of14C-ALA into chlorophyllawas not affected by 1 mM glyphosate. Thus, the site of action of glyphosate may involve two enzyme pathways:one controlling the conversion of α-ketoglutarate to ALA, and the other controlling the condensation of glycine with succinyl CoA to form ALA and carbon dioxide. Inhibition of ALA synthesis blocks synthesis of chlorophyll, as well as all other porphyrin ring compounds found in higher plants. Thus, inhibition of ALA synthesis may be an integral component of the herbicidal mode of action of glyphosate.

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The behaviour of plants in various gas mixtures

Proceedings of the Royal Society of London Series B Biological Sciences ◽

10.1098/rspb.1971.0089 ◽

1971 ◽

Vol 179 (1056) ◽

pp. 177-188

Keyword(s):

Carbon Dioxide ◽

Gas Exchange ◽

Partial Pressure ◽

Higher Plants ◽

Dry Mass ◽

Carbon Dioxide Partial Pressure ◽

Ambient Oxygen ◽

Leaf Dry Mass ◽

Rate Of Photosynthesis ◽

Effect Of Oxygen

The effects of the composition and pressure of the ambient gas mixture on the diffusive gas exchange of leaves, and the effects of carbon dioxide and oxygen on respiration and photosynthesis are described. When photosynthesis is limited by the rate at which carbon dioxide reaches the chloroplasts, the net rate of photosynthesis of many (but not all) plant species depends on the ambient oxygen partial pressure. The effect of oxygen may be principally to stimulate a respiratory process rather than to inhibit carboxylation. However, when photosynthesis is not limited by the carbon dioxide supply, this respiratory process seems to be suppressed. The gas exchange of plant communities responds to the aerial environment in the way expected from measurements on single leaves, but the growth response to a given difference in gas composition is smaller than expected because of adaptation, notably in the ratio of leaf dry mass to leaf area. It is concluded that the growth rate of higher plants in given illumination will be independent of the partial pressure of oxygen and of other gases likely to be used to dilute it, provided that the carbon dioxide partial pressure is so adjusted (probably to not more than 2 mbar (200 Pa)) that the rate of photosynthesis is not limited by the rate of diffusion to the chloroplasts.

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